1. Field of the Invention
The present invention relates to a stress evaluation apparatus for evaluating the physical property of a substance such as semiconductor and more particularly, it relates to a stress evaluation apparatus for evaluating internal stress existing in the substance.
2. Description of the Prior Art
In a conventional method of evaluating stress of a semiconductor member or the like which is manufactured through a process involving thermal expansion and internally provided with thermal stress, employed is a stress evaluation apparatus such as that disclosed in "Applied Physics", The Japan Society of Applied Physics, Vol. 50, No. 1, 1981; "Characterization of Semiconductors by Laser-Raman Spectroscopy" p. 69.
FIG. 1 is a schematic block diagram showing a conventional stress evaluation apparatus for measuring the Raman spectrum of scattered light.
Referring to FIG. 1, a light source 1 supplys excitation light such as Ar.sup.+ laser beam or He-Ne laser beam. Excitation light 9 supplied from the light source 1 is reflected by a mirror 10, and converged by a lens 11a on a measuring point of a substance 3 to be evaluated. Scattered light 12 from the measuring point of the substance 3 to be evaluated is converged by an another lens 11b, and spectro-analyzed by a spectroscope 13 such as a double monochromator. Spectro-analyzed light is detected by a detector 14, inputted in a microcomputer 15, and transmitted to a recorder 16.
In the conventional stress evaluation apparatus of the above structure, the Raman spectrum is measured in the following manner: The excitation light 9 supplied from the light source 1 is changed in direction by the mirror 10 and focused by the lens 11a, to be converged on/applied to the measuring point of the evaluated substance 3. The scattered light 12 from the measuring point of the evaluated substance 3 passes through the lens 11b, to be converged on an entrance slit of the spectroscope 13. The light is spectro-analyzed by the spectroscope 13 so that the Raman spectrum thereof is detected by the detector 14, to be inputted in the microcomputer 15 as an electric signal and stored in the same. Then the electric signal is subjected to data processing by the microcomputer 15 and transmitted to the recorder 16, which in turn records the Raman spectrum as a waveform or a peak wave number thereof.
Measurement of the Raman spectrum through the aforementioned apparatus is now described with reference to a flow chart. FIG. 2 is a flow chart showing conventional Raman spectrum measurement processing.
A substance to be evaluated is set at a step 601 and an optical system including lenses, mirror etc. is adjusted at a step 602, while conditions for Raman spectrum measurement are set at a step 603. Then, the Raman spectrum of light scattered from the evaluated substance is measured at a step 604. Description is now made on the routine of the Raman spectrum measurement processing with reference to another flow chart.
FIG. 3A is a flow chart showing a spectrum measurement routine in case of employing a photomultiplier as the detector 14.
First, the wave number of a spectroscope is set at a measurement start wave number .omega..sub.1 at a step 701. Referring to a step 702, Raman scattering intensity corresponding to the wave number is measured. In this case, intensity of Raman scattered light is converted into a voltage signal by the photomultiplier, to be measured. At a step 703, data on the Raman scattering intensity thus obtained and the set wave number of the spectroscope are transferred to a microcomputer. Referring to a step 704, these data are A-D converted in the microcomputer to be stored in a memory as digital signals. At a step 705, the wave number of the spectroscope is compared with a measurement end wave number .omega..sub.2, so that the wave number of the spectroscope is increased by .DELTA..omega. at a step 706 if the same is smaller than the measurement end wave number .omega..sub.2. Measurement of Raman scattering intensity corresponding to each wave number is repeated as shown in FIG. 3B until the wave number of the spectroscope exceeds the measurement end wave number .omega..sub.2, to be stored in the memory of the microcomputer as a digital signal. When the wave number of the spectroscope exceeds the measurement end wave number .omega..sub.2, the process is advanced to processing as shown in FIG. 2. At a step 605, a spectral waveform is outputted to a recorder on the basis of the data stored in the memory. Thereafter a peak wave number is read from the outputted spectral waveform at a step 606. Although the peak wave number is read from the recorded spectral waveform by an operator, the peak wave number value may be calculated by the microcomputer to be outputted to the recorder.
Description is now made on a method of evaluating internal stress existing in a substance from the Raman spectrum measured in the aforementioned manner. Raman scattered light results from excitation light striking the evaluated substance and partially losing its energy as vibration energy for component atoms and molecules etc. of the substance, to be different in wavelength from the original excitation light. The energy variation corresponds to the energy of lattice vibration and molecule vibration of the evaluated substance, and depends on stress existing therein. This variation corresponds to change in wave number in a peak of the measured Raman spectrum. FIG. 4 shows such a phenomenon with respect to silicon, for example. Referring to FIG. 4, a one-dot chain line 80 denotes the peak wave number of the Raman spectrum of single crystal silicon having no stress, which peak wave number is 520.5 cm.sup.-1. However, in case of silicon internally having stress such as SOI (silicon on insulator: polysilicon deposited on silicon oxide) structure recrystallized by irradiation of laser beam, the peak number of its Raman spectrum as measured is shifted to a lower wave number side as shown by a solid line 81. This is because tensile stress exists in the SOI structure. Further, compressive stress exists in SOS (silicon on sapphire), which is polysilicon deposited on a sapphire substrate, and hence the peak wave number thereof is shifted to a higher wave number side as shown by a dotted line 82.
Thus, stress existing in a substance is evaluated through the fact that the stress corresponds to variation in peak wave number of the Raman spectrum.
As hereinabove described, stress existing in a substance is generally evaluated through difference between peak numbers of Raman spectra of a substance having no stress and the same substance internally having stress. However, such a value is influenced not only by the value of the stress but also by temperature difference in the substance. The results of measurement of relation between the peak wave numbers of Raman bands of silicon samples and sample temperatures are described in Physical Review B, Vol. 1, No. 2, pp. 638-642 (1970): "Temperature Dependence of Raman Scattering in Silicon" and Applied Physics Letters, Vol. 41(11), pp. 1016-1018 (1982): "Temperature Dependence of Silicon Raman Lines". For example, when the output power of excitation light applied to the same measuring point of an evaluated substance is changed, the peak wave number of the measured Raman spectrum is varied as shown in FIG. 5. This is because the temperature of the evaluated substance is varied with the output power of the excitation light. Such a phenomenon may occur in case of evaluating a substance having sectional structure as shown in FIG. 6, even if excitation light of the same output power is employed. When a silicon thin film 101 is irradiated with excitation light, temperature rise by the irradiation is increased since a silicon oxide film 102 has low thermal conductivity. When a silicon substrate 103 is irradiated with the excitation light of the same output power, heat by the irradiation is diffused in the interior of the silicon substrate 103 and hence temperature rise thereof is small as compared with that of the silicon thin film 101. Therefore, even if the silicon thin film 101 internally has stress of the same level as the silicon substrate 103, difference is caused in peak wave numbers of the Raman spectra employed for stress evaluation since the two members are different in temperature rise by irradiation from each other.